The present invention relates to a high performance hollow cathode discharge light source, more particularly to a high performance hollow cathode lamp which is used, for example, in atomic absorption spectral analysis or atomic fluorescence spectral analysis.
At present, the principal light source in atomic absorption spectral analysis is a conventional hollow cathode lamp in which the cathode is in the form of a hollow cylinder with one open end, and is made of a material which contains elements from which the atomic spectrum is to be obtained. As disclosed in U.S. Pat. No. 3,264,511, a hollow cathode and an anode are sealed in an envelope and are positioned at the axis of the lamp and spaced from each other. At the other end of the lamp opposite to the cathode and anode there is a window. All of those parts are in an atmosphere of an inert gas contained in the envelope. The electric discharge between the anode and cathode serves to generate an atomic vapour by sputtering from the cathode, and also to supply excitation, which is necessary for the production of atomic spectra, to at least some of the atoms in the vapour. Such a lamp suffers from the principal disadvantages that those two functions, sputtering and excitation of discharge cannot be controlled separately and a variation in any parameter of the discharge, e.g. current or pressure, will affect both functions. The amount of atomic vapour produced thereby, therefore, must be restricted to a relatively small degree if it is not desired to increase the widths of the spectral lines by self-absorption and resonance broadening. Thus the available discharge current and the degree of excitation that can be imparted to the atomic vapour are similarly limited. Consequently, the intensities of the spectra emitted by such discharge lamp have to be restricted if sharp lines are required.
Both U.S. Pat. Nos. 3,433,963 and 3,517,188 disclose a Sullivan-Walsh type high intensity atomic lamp respectively. Z. Van Gelder described a modified version of the Sullivan-Walsh lamp in "Appl. spectrosc. (22)", P. 581, 1986. U.S. Pat. No. 4,158,790 discloses another kind of high intensity atomic spectral lamp with interchangeable cathode. Those high intensity hollow cathode lamps involve two separate discharge proceedings. One is hollow cathode discharge which produces atomic vapour by sputtering from the internal walls of the hollow cathode and excites thereby part of atomic vapour in negative glow region. The other is low voltage high current discharge which gives rise to further excitation of the sputtered atomic vapour. All of those are provided with a thermal filament auxiliary cathode coated with oxide film from which electrons may be easily emitted so as to produce a large discharge current at low voltage. Taking advantage of such structure, the high intensity hollow cathode lamps above are of high intensity light output, usually four to seventeen times higher than conventional cathode lamps or Sullivan-walsh type, but complicated in structure. They usually require three power supplies. "A high-intensity hollow-cathode lamp for atomic fluorescence", Spectrochemica Acta, vol. 26B, pp. 201-205 (1971).
R. M. Lowe discloses a Lowe type high intensity hollow cathode discharge lamp which includes an envelope, an open-ended cylindrical hollow cathode as primary cathode; an oxide-coated thermal filament cathode as auxiliary or booster cathode and a common anode. Above and below the cathode cylinder are mounted plate discharge restricting baffles with only a central hole. The function of these baffles, is to channel the secondary discharge from the oxide-coated cathode above the primary cathode cylinder, through the centre of the cathode cylinder, to the common anode below. Like other high intensity hollow cathode lamps, it requires three power supplies as well. One is a Low-voltage A.C. power supply for heating thermal filament booster cathode; one is a low-voltage high-current discharge power supply for the discharge between the booster cathode and the common anode; and another is primary hollow cathode discharge power supply. Due to the complicated structure, those lamps are difficult to manufacture an high in cost.